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Univ. of Sistsn and Baluchestan ٠۵/٠٢/١۴۴٠ Ali Ebrahimi ١

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Univ. of Sistsn and Baluchestan ٠۵/٠٢/١۴۴٠ Ali Ebrahimi ١ ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan Ira N. Levine, Quantum Chemistry + MOLECULAR ORBITALS FOR H2 EXCITED STATES we construct approximate functions for further excited + states so as to build up a supply of H2 -like molecular orbitals. We shall then use these MOs to discuss many-electron diatomic molecules qualitatively, (just as hydrogenlike AOs). we can use the linear-variation-function method: we add in more AOs to the previous linear combination. + for the six lowest H2 σ states, three lowest m = 0 hydrogenlike functions For the symmetry of the homonuclear diatomic molecule + : even (g) states _ : odd (u) states ١ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan The relative magnitudes of the coefficients: For the two states that dissociate into a 1s hydrogen atom: c1 >> c2 and c3 (c2 and c3 vanish in the limit of R going to infinity) As a first approximation, LCAO functions From the viewpoint of perturbation theory: if we take the separated atoms as the unperturbed problem, the above functions are the correct zeroth-order wave functions. for the two states that dissociate to a 2s hydrogen atom c2 >> c1 and c3 (c1 and c3 vanish in the limit of R going to infinity) As a first approximation, LCAO functions To find rigorous upper bounds to the energies of these + two H2 states, we must use the upper trial function and solve the appropriate secular equation contour diagrams ٢ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan σ MOs even though they correlate with 2p separated AOs, since they have m = 0. dashed lines ≡ nodal surfaces For the hydrogen atom, the 2s and 2p AOs are degenerate, and so … + In the R-> limit, H2 consists of an H atom perturbed by the essentially uniform electric field of a far-distant proton. for the n = 2 levels the correct zeroth-order functions For molecules that dissociate into many-electron atoms, the separated- atoms 2s and 2p AOs are not degenerate but do lie close together in energy. Hence, … ٣ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan For the other two 2p atomic orbitals, we can use either the 2p+1 and 2p-1 complex functions or the 2px and 2py real functions. + inversion of the electron's coordinates in H2 Ali Ebrahimi ۴ ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan inversion of the electron's coordinates a u orbital. Reflection in the plane perpendicular to the axis and midway between the nuclei: unchanged, an unstarred (bonding) orbital. ≡ Ali Ebrahimi ۵ ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan ≡ the λ = 1 energy levels are doubly degenerate, corresponding to m = 1 have the same shapes, do not give charge buildup between the nuclei Ali Ebrahimi ۶ ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan the more familiar alternatives: builds up probability density is not symmetrical nodal plane about z Bonding (u) antibonding (g) and have the same energy. are not eigenfunctions of Lz linear combinations of are eigenfunctions of Lz and have the same energy. are not eigenfunctions of Lz linear combinations of are eigenfunctions of Lz ٧ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan + how H2 MOs correlate with the united-atom AOs. R 0 R 0 p united-atom states R 0 d united-atom states ? MO CONFIGURATIONS OF HOMONUCLEAR DIATOMIC MOLECULES If we ignore the interelectronic repulsions, the zeroth- + order wave function is a Slater determinant of H2 -like one-electron spin-orbitals. + We approximate the spatial part of the H2 spin-orbitals by the LCAO-MOs The sizes and energies of the MOs vary with varying internuclear distance for each molecule and vary as we go from one molecule to another. the order in which the MOs fill as we go across the periodic table: ٨ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan Molecular-Orbital Nomenclature for Homonuclear Diatomic Molecules the noncrossing rule: the energies of MOs with the same symmetry cannot cross. symmetry refers to whether the orbital is g or u and whether it is σ, π, δ.... s, d, g,... united-atom AOs (even) correlate with g MOs, p, l, h,... AOs (odd) correlate with u MOs. Correlation diagram for homonuclear diatomic MOs. this diagram is not quantitative. ٩ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan Homonuclear diatomic MOs formed from 1s, 2s, and 2p AO's. Molecular electronic configurations terms, levels, and states + H2 De = 4.75 eV H2 no net bonding, He2 It has bound excited electronic states, about two dozen such bound excited states of He2 have been spectroscopically observed in gas discharge tubes. such excited states decay to the ground electronic state, and then the molecule dissociates. + He2 ١٠ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan Li2 a single bond indicate the negligible change in inner- shell orbital energies on molecule formation no net bonding Be2 electrons. two net bonding B2 electrons one electron in the MO and the other in the MO, is at variance with the notion that single bonds are always σ bonds. the electron-spin-resonance spectrum showed that the ground term is a triplet with S = 1 C2 with four net bonding electrons the and MOs have nearly the same energy in many molecules. giving a triplet term. a singlet term. the ground term by a small margin (0.09 eV) N2 a triple bond O2 a double bond. Spectroscopic evidence indicates that in O2 (and in F2) the σg2p is lower in energy than the πu2p MO. The paramagnetism of O2 ١١ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan a single bond. F2 Ne2 no net bonding electrons Na2 0.02 eV below Al2 0.05 eV below Si2 the harmonic vibrational frequency ١٢ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan Bonding MOs produce charge buildup between the nuclei, whereas antibonding MOs produce charge depletion between the nuclei. Usually, removal of an electron from: a bonding MO decreases De an antibonding MO increases De + N2 → N2 : the dissociation energy decreases + O2 → O2 : the dissociation energy increases Energy is always required to ionize a stable molecule, no matter which electron is removed. Are interaction between two ground-state He atoms strictly repulsive ? helium gas can be liquefied ! van der Waals forces: all kinds of intermolecular forces. For example: London or dispersion force Except for highly polar molecules: the dispersion force is the largest contributor to intermolecular attractions. van der Waals molecules Ar2 van der Waals molecules: De = 0.012 eV and Re = 3.76 A; Ar2 has seven bound vibrational levels (v = 0,..., 6). ١٣ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan Examples of diatomic van der Waals molecules: Re, De Ne2 : 3.1 Å, 0.0036 eV HeNe : 3.2 Å, 0.0012 eV Ca2 : 4.28 Å, 0.13 eV Mg : 2,3.89 Å, 0.053 eV ELECTRONIC TERMS OF DIATOMIC MOLECULES For atoms, each set of degenerate atomic orbitals constitutes an atomic subshell. For molecules, each set of degenerate molecular orbitals constitutes a molecular shell. one MO Two MOs a molecular electronic configuration giving the number of electrons in each shell ١۴ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan For a many-electron diatomic molecules: Lz = MLħ, where ML = 0, ±1, ±2, ± …. 2 electronic energy depends on ML double degeneracy The magnitude of S : S = 0, 1/2, 1, 3/2, … The component of S along an axis = Msħ, where Ms = S, S - 1,..., -S. 2S + 1 is called the spin multiplicity 2S+1Λ Diatomic electronic states that arise from the same electron configuration and that have the same value for Λ and the same value for S are said to belong to the same electronic term. ١۵ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan for a filled-shell molecular configuration: Ms = ∑ ms values = 0 S = 0 A filled σ shell : ML = ∑m = 0 A filled π shell : ML = ∑m = (-1) + (+1) = 0 A filled δ, ϕ, … shell : ML = ∑m = 0 a closed-shell molecular configuration: S = 0 , Λ = 0 only a 1∑ term ground electronic configuration of H2 A single σ electron: s = ½ S = ½ → 2∑ A single π electron: s = ½ S = ½ → 2∏ …. more than one electron: Nonequivalent : Electrons that are in different molecular shells Equivalent : Electrons that are in same molecular shells two nonequivalent electrons: σσ configuration M = 0 1 3 L S = 1 or 0 ∑ and ∑ Each s = ½ σπ configuration: M = 1 1 3 L S = 1 or 0 ∏ and ∏ Each s = ½ ١۶ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan πδ configuration, π electron: m = M = +3, -3, +1, -1 Λ = 3 or 1 ±1 L δ electron: m = ±2 1∏, 3∏, 1Φ, 3Φ. ππ configuration, each electron ML has m = ±1 values 2, -2, 0, 0 Λ= 2,0, and 0 1Δ, 3Δ, 1∑, 3∑, 1∑, and 3∑ apart from spin degenerate states nondegenerate degeneracy: Forms of the wave functions for the terms two π subshells: π and π' M = +2 L a subscript indicates the m value For the Δ terms: both electrons have m = +1 or both have m = - 1. neither symmetric the spatial factor in nor antisymmetric the wave function is with respect to … and are unacceptable ١٧ Ali Ebrahimi ١۴۴٠/٠٢/٠۵ Univ. of Sistsn and Baluchestan both electrons symmetric having m = + 1 antisymmetric both electrons symmetric having m = - 1 antisymmetric symmetric or antisymmetric with respect to exchange of electron two states six states For the ∑ terms: one electron with m = +1 and one with m = - 1.
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